o Rename acpi_iommu_get_dma_tag() -> iommu_get_dma_tag().

This function isn't ACPI dependent and we may use it on FDT systems
as well.

o Don't repeat the function declaration, include iommu.h instead.

Reviewed by: andrew, kib
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D26584